With reference to FIG. 8, a prior art lens barrel as disclosed in Document 1 listed below comprises a fixed lens barrel 201 that has a raised portion 201a circularly extending along the entire outer circumferential surface and a longitudinal groove 201b in parallel with the optical axis. A fore lens frame 205 holds a fore group of component lens pieces (or a fore lens group) 204, and is provided with a projection 205a provided in the outer circumferential surface and passing through the groove 201b. 
A zooming ring (or a focusing ring) 202 is fitted on the outer circumferential surface of the fixed lens barrel 201 and the fore lens frame 205, and is rotatable about the optical axis. The zooming ring 202 includes a projected portion 202a radially extended to engage with the raised portion 201a, and a cam groove 202b in which a tip of the projection 205a is fitted. The cam groove 202b runs helically in an inner circumferential surface of the zooming ring 202, and rotating the zooming ring 202 in a circumferential direction permits the projection 205a to move along the optical axis. Since the projection 205a is fitted in and passed through longitudinal groove 201b, the fore lens frame 205 and the fore lens group 204 are capable of linearly moving along the optical axis relative to the fixed lens barrel 201.
A fixed ring 203 includes threads 210 engaged with a fore end of the fixed lens barrel 201, and a pusher 214 pressing against a wavy washer 212. The wavy washer 212 is pressed rearward from the side closer to the subject by the pusher 214, and this urges the projected portion 202a of the zooming (or focusing) ring 202 to move rearward under a predetermined pressing force. The projected portion 202a is squeezed between the wavy washer 212 and the raised portion 201a of the fixed lens barrel 201, namely, between two points in a line along the optical axis.
Arranged in such a manner, the lens barrel has the zooming ring 202 tightly abutted against the fixed lens barrel 201 without looseness along the optical axis, and the lens barrel enables an appropriate frictional force against the rotation force caused by manipulating rotary operation rings such as the zooming (or focusing) ring so as to rotate the zooming (or focusing) ring over the outer circumferential surface of the fixed lens barrel 201 with a torque imparted thereon in some predetermined perceptible degree.
Another prior art embodiment of the lens barrel is disclosed in Document 2 listed below. The lens barrel, as recognized in FIG. 9, includes a hollow cylinder-like lens barrel body, one or more movable lens frame(s) disposed inside the lens barrel body and slidable along the optical axis, a guide member guiding the movable lens frame along the optical axis, a cam ring 318 rotatable about an axis along the optical axis inside or outside the lens barrel body and provided with cam grooves 332 that serve to slide the movable lens frame along the optical axis in response to the rotation of the cam ring, and a plurality of pusher means 324 located in a fore or rear end of the cam ring 318 to press the cam ring 318 against the lens barrel body in a single direction along the optical axis. At any location of the fore or rear end of the cam ring 318 pressed by the pusher means 324 other than the point where a trajectory of any of the cam grooves 332 is closest to the fore or rear end, receptacles 344 are formed to drop and rest the pusher means 324 therein.
In still another prior art embodiment of the lens barrel, grease may be applied to junctions of the outer circumferential surface of the fixed barrel with the inner circumferential surface of the zooming (or focusing) ring so as to utilize a viscosity of the grease to impart torque in some perceptible degree.
Patent Document 1: Japanese Patent Preliminary Publication No. 2001-305409
Patent Document 2: Japanese Patent Preliminary Publication No. 2000-180689
Since the prior art lens barrel disclosed in Document 1 is provided with a plurality of operation rings, there must be the wavy washers as many as the operation rings to press against the projected portion of the operation rings. Thus, the wavy washers as many as the operation rings desired to impart some appropriate touch of torque must be available in advance. In addition, in order to attain the optimum torque or operation force in manipulating any of the operation rings, namely, in order to impart the torque by an adequate touch, a variety of elastic members, or a variety of wave washers in this case, of various angles and/or various dimensions must be prepared. This brings about an increase in the manufacturing cost as well as the complicatedness of maintaining each component. In fabricating the lens barrel, using such a wider variety of wave washers is prone to lead to troubles during the assembling and the increased number of essential components.
In the variation of the exemplary lens barrel disclosed in Document 2, elastic members in FIGS. 4 and 5 of Document 2, namely, resilient members 46A, 46C and a beam 46D are integrally formed. Such an integrated elastic element of the resilient members 46A, 46C and the beam 46D is advantageous in reducing the number of components as well as the steps of assembly, but the issue left unimproved is that the elastic element must be still required for each of the operation rings. Another issue also left unimproved is that a variety of elastic members, or a variety of wave washers in this case, of various angles and/or various dimensions must be prepared in order to attain an appropriate touch of torque on the operation rings.
When grease is applied to junctions of the outer circumferential surface of the fixed barrel with the inner circumferential surface of any of the operation rings so as to utilize a viscosity of the grease to impart torque in some perceptible degree, an amount of the applied grease and a variation in temperature vary a reaction force of manipulation. Thus, the touch of the torque imparted on the operation ring is unstable, and the grease adversely infiltrates into surfaces of the component lens pieces as a temperature rises.
The present invention is made to overcome the aforementioned disadvantages of the prior art embodiments of the lens barrel, and accordingly, it is an object o the present invention to provide a lens barrel that enables an appropriate frictional force against the rotation force caused by manipulating rotary operation rings such as a zooming ring and a focusing ring to effect the optimum operating force on the rotary operation rings. Thus, since a single resilient element is used to press simultaneously against two of the operation rings, the lens barrel can reduce the number of the components and the steps of assembly and can effectively attain downsizing and cost reduction, compared with a case where two of the operation rings are associated with their respective resilient elements to impart a torque on the operation rings.
It is another object of the present invention to provide a lens barrel capable of retaining a constant operation torque, namely, an operation force regardless of a variation in atmospheric temperature.